This application pertains to the art of manufacturing plastic tubing and more particularly to the art of manufacturing corrugated semiround plastic tubing, conduit, or pipe. The invention is particularly applicable to corrugated plastic drainage tubing and will be described with particular reference thereto. It will be appreciated, however, that the invention has broader applications, such as manufacturing septic and leach bed tubings, subterranean conduit, other fluid conveying channels, pipes for shielding electrical conduit, and the like.
In the past, most corrugated drainage tubing has been circular in cross-section. The circular tubing is generally manufactured in linear production line assembly. Commonly, a plastic extruding machine fitted with a circular extrusion die receives pellets of thermoplastic material and extrudes a plastic sleeve through the die. The extruder subjects the pellets to a combination of heat and pressure to soften the pellets to a plastic state. Closely adjacent the extruder die is positioned a continuous blow molding assembly. Often the blow molding assembly includes pairs of mold blocks arranged in two continuous loops. Adjacent the extruding die, each pair of mold blocks abuts together to define a circular corrugated mold. The mold blocks then travel linearly along a path as the sleeve is forced with pneumatic pressure to conform to the shape of the interior surface of the mold blocks. While the mold blocks of each pair are abutting each other and traveling in contact with the tubing, the plastic material cools sufficiently that it retains its molded shape. The mold blocks then separate and return to the initial position.
After the tubing is molded, it is generally cooled with a water bath to make the plastic sufficiently hard for slotting or drilling operations. If the tubing is to be used for drainage tubing, the continuous length of tubing is commonly conveyed adjacent a slotting saw which cuts a series of thin clots into the side walls of the tubing. If the tubing is to be used for septic tubing, the tubing is commonly conveyed through a drilling machine which drills round holes about 3/4" in diameter into the side walls of the tubing. If the tubing is to be used for conveying fluids, generally, it is not slotted or drilled. Further, if the tubing is to be used for drainage tubing, it is sometimes wrapped with a filter material. This filter material may be applied in several manners. One common method of application is to wrap the tubing from a continuous sheet of filter material and seal the edges of the filter material together such that tubing is encased within a sleeve of the filter material. The plastic tubing, with or without the filter wrap, is commonly cut in ten foot lengths or coiled in 250 foot rolls. An alternate method for applying filter material to corrugated plastic tubing is to insert the end of the tubing into a bunched 250 foot sleeve of filter material. As the tubing is coiled, the sleeve is unbunched and fed smoothly over the tubing. Alternately filter material can be applied as the tubing is installed in the ground. Numerous patents illustrate the conventional methods for the manufacture of corrugated plastic tubing. See for example, U.S. Pat. Nos. 3,732,046, issued May 8, 1973 to R. C. Martin; 3,819,778, issued June 25, 1974 to E. J. Maroschak; 3,843,758, issued Oct. 22, 1974 to E. J. Maroschak; and 3,990,827, issued Nov. 9, 1976 to E. J. Maroschak.
One of the problems with the prior art apparatus and method for manufacturing corrugated plastic tubing is that only a single length of tubing is manufactured on each production line.
Another problem with the prior art manufacturing techniques is the difficulty encountered when drilling or slotting moving lengths of continuous plastic tubing.
Yet another problem with the prior art manufacturing apparatus and method is the difficulty encountered in adding and handling filter materials to the tubing. Filter wrapped materials applied during manufacture to the exterior of the tubing are very vulnerable to abrasion and puncture during storage, shipment, and installation. Application of filter material at installation, see for example U.S. Pat. No. 4,003,122, issued Jan. 18, 1977 to R. C. Overmyer, makes the installation process slower and more cumbersome.